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2019-07-18implement fstatat with SYS_statx, conditional on undersized kstat timeRich Felker1-2/+68
this commit adds a new backend for fstatat (and thereby the whole stat family) using the SYS_statx syscall, but conditions the new code on the kernel stat structure's time fields being smaller than time_t. in principle that should make it all dead code at present, but mips64 has a broken stat structure with 32-bit time fields despite having 64-bit time_t elsewhere, so on mips64 it is a functional change that makes post-Y2038 filesystem timestamps accessible. whenever the 32-bit archs end up getting 64-bit time_t, regardless of how that happens, the changes in this commit will automatically take effect for them too.
2019-07-18cleanup includes now that stat, lstat no longer make direct syscallsRich Felker2-2/+0
2019-07-18restore property that fstat(AT_FDCWD) fails with EBADFRich Felker1-0/+1
AT_FDCWD is not a valid file descriptor, so POSIX requires fstat to fail with EBADF. if passed to fstatat, the call would spuriously succeed and return results for the working directory.
2019-07-18decouple struct stat from kernel typeRich Felker1-4/+27
presently, all archs/ABIs have struct stat matching the kernel stat[64] type, except mips/mipsn32/mips64 which do conversion hacks in syscall_arch.h to work around bugs in the kernel type. this patch completely decouples them and adds a translation step to the success path of fstatat. at present, this is just a gratuitous copying, but it opens up multiple possibilities for future support for 64-bit time_t on 32-bit archs and for cleaned-up/unified ABIs. for clarity, the mips hacks are not yet removed in this commit, so the mips kstat structs still correspond to the output of the hacks in their syscall_arch.h files, not the raw kernel type. a subsequent commit will fix this.
2019-07-18refactor all stat functions in terms of fstatatRich Felker4-23/+37
equivalent logic for fstat+O_PATH fallback and direct use of stat/lstat syscalls where appropriate is kept, now in the fstatat function. this change both improves functionality (now, fstatat forms equivalent to fstat/lstat/stat will work even on kernels too old to have the at functions) and localizes direct interfacing with the kernel stat structure to one file.
2019-07-16fix broken lseek on x32 (x86_64/ILP32) with offsets larger than LONG_MAXRich Felker1-0/+15
this is analogous to commit 918c5fa0fc656e49b1ab9ce47183a23e3a36bc00 which fixed the corresponding issue for mips n32.
2019-07-16fix broken lseek on mipsn32 with offsets larger than LONG_MAXRich Felker1-0/+20
mips n32 has 32-bit long, and generally uses long syscall arguments and return values, but provides only SYS_lseek, not SYS_llseek. we have some framework (syscall_arg_t, added for x32) to make syscall arguments 64-bit in such a setting, but it's not clear whether this could match the sign-extension semantics needed for 32-bit args to all the other syscalls, and we don't have any existing mechanism to allow the return value of syscalls to be something other than long. instead, just provide a custom mipsn32 version of the lseek function doing its own syscall asm with 64-bit arguments. as a result of commit 03919b26ed41c31876db41f7cee076ced4513fad, stdio will also get the new code, fixing fseeko/ftello too.
2019-07-16use namespace-safe __lseek for __stdio_seek instead of direct syscallRich Felker3-10/+6
this probably saves a few bytes, avoids duplicating the clunky lseek/_llseek syscall convention in two places, and sets the stage for fixing broken seeks on x32 and mipsn32.
2019-07-13fix sigaltstack to ignore ss_size with SS_DISABLE, per POSIXJames Y Knight1-1/+1
2019-07-10use the correct attributes for ___errno_locationSamuel Holland1-0/+3
In the public header, __errno_location is declared with the "const" attribute, conditional on __GNUC__. Ensure that its internal alias has the same attributes. Maintainer's note: This change also fixes a regression in quality of code generation -- multiple references to errno in a single function started generating multiple calls again -- introduced by commit e13063aad7aee341d278d2a879a76ec7b59b2ad8.
2019-07-10fix restrict violations in internal use of several functionsSamuel Holland3-10/+10
The old/new parameters to pthread_sigmask, sigprocmask, and setitimer are marked restrict, so passing the same address to both is prohibited. Modify callers of these functions to use a separate object for each argument.
2019-07-08prevent dup2 action for posix_spawn internal pipe fdRich Felker1-0/+4
as reported by Tavian Barnes, a dup2 file action for the internal pipe fd used by posix_spawn could cause it to remain open after execve and allow the child to write an artificial error into it, confusing the parent. POSIX allows internal use of file descriptors by the implementation, with undefined behavior for poking at them, so this is not a conformance problem, but it seems preferable to diagnose and prevent the error when we can do so easily. catch attempts to apply a dup2 action to the internal pipe fd and emulate EBADF for it instead.
2019-07-02remove stray .end directives from powerpc[64] asmFangrui Song2-2/+0
maintainer's note: these are not meaningful/correct/needed and the clang integrated assembler errors out upon seeing them.
2019-07-01ipc: prefer SYS_ipc when it is definedSzabolcs Nagy12-12/+12
Linux v5.1 introduced ipc syscalls on targets where previously only SYS_ipc was available, change the logic such that the ipc code keeps using SYS_ipc which works backward compatibly on older kernels. This changes behaviour on microblaze which had both mechanisms, now SYS_ipc will be used instead of separate syscalls.
2019-07-01fix deadlock in synccall after threaded forkSamuel Holland1-0/+1
synccall may be called by AS-safe functions such as setuid/setgid after fork. although fork() resets libc.threads_minus_one, causing synccall to take the single-threaded path, synccall still takes the thread list lock. This lock may be held by another thread if for example fork() races with pthread_create(). After fork(), the value of the lock is meaningless, so clear it. maintainer's note: commit 8f11e6127fe93093f81a52b15bb1537edc3fc8af and e4235d70672d9751d7718ddc2b52d0b426430768 introduced this regression. the state protected by this lock is the linked list, which is entirely replaced in the child path of fork (next=prev=self), so resetting it is semantically sound.
2019-06-28cap getdents length argument to INT_MAXRich Felker1-0/+2
the linux syscall treats this argument as having type int, so passing extremely long buffer sizes would be misinterpreted by the kernel. since "short reads" are always acceptable, just cap it down. patch based on report and suggested change by Florian Weimer.
2019-06-25allow fmemopen with zero sizeRich Felker1-1/+1
previously, POSIX erroneously required this to fail with EINVAL despite the traditional glibc implementation, on which the POSIX interface was based, allowing it. the resolution of Austin Group issue 818 removes the requirement to fail.
2019-06-21do not use _Noreturn for a function pointer in dynamic linkerMatthew Maurer1-1/+1
_Noreturn is a C11 construct, and may only be used at the site of a function definition.
2019-06-14add riscv64 architecture supportRich Felker24-0/+465
Author: Alex Suykov <alex.suykov@gmail.com> Author: Aric Belsito <lluixhi@gmail.com> Author: Drew DeVault <sir@cmpwn.com> Author: Michael Clark <mjc@sifive.com> Author: Michael Forney <mforney@mforney.org> Author: Stefan O'Rear <sorear2@gmail.com> This port has involved the work of many people over several years. I have tried to ensure that everyone with substantial contributions has been credited above; if any omissions are found they will be noted later in an update to the authors/contributors list in the COPYRIGHT file. The version committed here comes from the riscv/riscv-musl repo's commit 3fe7e2c75df78eef42dcdc352a55757729f451e2, with minor changes by me for issues found during final review: - a_ll/a_sc atomics are removed (according to the ISA spec, lr/sc are not safe to use in separate inline asm fragments) - a_cas[_p] is fixed to be a memory barrier - the call from the _start assembly into the C part of crt1/ldso is changed to allow for the possibility that the linker does not place them nearby each other. - DTP_OFFSET is defined correctly so that local-dynamic TLS works - reloc.h LDSO_ARCH logic is simplified and made explicit. - unused, non-functional crti/n asm files are removed. - an empty .sdata section is added to crt1 so that the __global_pointer reference is resolvable. - indentation style errors in some asm files are fixed.
2019-05-26optimize aarch64 dynamic tlsdesc function to spill fewer registersRich Felker1-10/+7
with the glibc generation counter model for reusing dynamic tls slots after dlclose, it's really not possible to get away with fewer than 4 working registers. for us however it's always been possible, but tricky, and only became apparent after the switch to installing new dynamic tls at dlopen time. by merging the negated thread pointer into the addend early, the register holding the thread pointer can immediately be reused, bringing the working register count down to three. this allows saving/restoring via a single stp/ldp pair, since the return register x0 does not need to be saved. net reduction of 3 instructions, 2 of which were push/pop.
2019-05-16fix tls offsets when p_vaddr%p_align != 0 on TLS_ABOVE_TP targetsSzabolcs Nagy1-1/+2
currently the bfd linker does not seem to create tls segments where p_vaddr%p_align != 0, but this is valid in ELF and then the runtime computed tls offset must satisfy offset%p_align == (base+p_vaddr)%p_align and in case of local exec tls (main executable) the smallest such offset must be used (otherwise it is incompatible with the offset computed by the static linker). the !TLS_ABOVE_TP case is handled correctly (the offset is negative then in the formula). the ldso code for TLS_ABOVE_TP is changed so the static tls offset of each module satisfies the formula.
2019-05-16fix format strings for uid/gid values in putpwent/putgrentRich Felker2-2/+2
commit 648c3b4e18b2ce2b6af7d44783e42ca267ea49f5 omitted this change, which is needed to be able to use uid/gid values greater than INT_MAX with these interfaces. it fixes alpine linux bug #10460.
2019-05-05make fgetwc set error indicator for stream on encoding errorsRich Felker1-2/+8
this is a requirement in POSIX that's omitted, and seemed potentially non-conforming, in the C standard. as such it was omitted here. however, as part of Austin Group issue #1170, the discrepancy was raised with WG14 and determined to be unintended; future versions of the C standard will require the error indicator to be set, as POSIX does.
2019-05-05allow archs to provide a 7-argument syscall if neededRich Felker1-0/+1
commit 788d5e24ca19c6291cebd8d1ad5b5ed6abf42665 noted that we could add this if needed, and in fact it is needed, but not for one of the archs documented as having a 7th syscall arg register. rather, it's needed for mips (o32), where all but the first 4 arguments are passed on the stack, and the stack can accommodate a 7th.
2019-04-20make new math code compatible with unused variable warning/errorRich Felker1-3/+6
commit b50d315fd23f0fbc4c11e2583801dd123d933745 introduced fp_force_eval implemented by default with a dead store to a volatile variable. unfortunately introduces warnings with -Wunused-variable and breaks the ability to use -Werror with the default warning options set by configure when warnings are enabled. we could just call fp_barrier instead, but that results in a spurious load after the store due to volatile semantics. the fix committed here avoids the load. it will still produce warnings without -Wno-unused-but-set-variable, but that's part of our default warning profile, and there are already other locations in the source where an unused variable warning will occur without it.
2019-04-17math: new powSzabolcs Nagy4-303/+521
from https://github.com/ARM-software/optimized-routines, commit 04884bd04eac4b251da4026900010ea7d8850edc The underflow exception is signaled if the result is in the subnormal range even if the result is exact. code size change: +3421 bytes. benchmark on x86_64 before, after, speedup: -Os: pow rthruput: 102.96 ns/call 33.38 ns/call 3.08x pow latency: 144.37 ns/call 54.75 ns/call 2.64x -O3: pow rthruput: 98.91 ns/call 32.79 ns/call 3.02x pow latency: 138.74 ns/call 53.78 ns/call 2.58x
2019-04-17math: new exp and exp2Szabolcs Nagy4-480/+434
from https://github.com/ARM-software/optimized-routines, commit 04884bd04eac4b251da4026900010ea7d8850edc TOINT_INTRINSICS and EXP_USE_TOINT_NARROW cases are unused. The underflow exception is signaled if the result is in the subnormal range even if the result is exact (e.g. exp2(-1023.0)). code size change: -1672 bytes. benchmark on x86_64 before, after, speedup: -Os: exp rthruput: 12.73 ns/call 6.68 ns/call 1.91x exp latency: 45.78 ns/call 21.79 ns/call 2.1x exp2 rthruput: 6.35 ns/call 5.26 ns/call 1.21x exp2 latency: 26.00 ns/call 16.58 ns/call 1.57x -O3: exp rthruput: 12.75 ns/call 6.73 ns/call 1.89x exp latency: 45.91 ns/call 21.80 ns/call 2.11x exp2 rthruput: 6.47 ns/call 5.40 ns/call 1.2x exp2 latency: 26.03 ns/call 16.54 ns/call 1.57x
2019-04-17math: new log2Szabolcs Nagy3-106/+335
from https://github.com/ARM-software/optimized-routines, commit 04884bd04eac4b251da4026900010ea7d8850edc code size change: +2458 bytes (+1524 bytes with fma). benchmark on x86_64 before, after, speedup: -Os: log2 rthruput: 16.08 ns/call 10.49 ns/call 1.53x log2 latency: 44.54 ns/call 25.55 ns/call 1.74x -O3: log2 rthruput: 15.92 ns/call 10.11 ns/call 1.58x log2 latency: 44.66 ns/call 26.16 ns/call 1.71x
2019-04-17math: new logSzabolcs Nagy3-104/+454
from https://github.com/ARM-software/optimized-routines, commit 04884bd04eac4b251da4026900010ea7d8850edc Assume __FP_FAST_FMA implies __builtin_fma is inlined as a single instruction. code size change: +4588 bytes (+2540 bytes with fma). benchmark on x86_64 before, after, speedup: -Os: log rthruput: 12.61 ns/call 7.95 ns/call 1.59x log latency: 41.64 ns/call 23.38 ns/call 1.78x -O3: log rthruput: 12.51 ns/call 7.75 ns/call 1.61x log latency: 41.82 ns/call 23.55 ns/call 1.78x
2019-04-17math: new powfSzabolcs Nagy4-240/+232
from https://github.com/ARM-software/optimized-routines, commit 04884bd04eac4b251da4026900010ea7d8850edc POWF_SCALE != 1.0 case only matters if TOINT_INTRINSICS is set, which is currently not supported for any target. SNaN is not supported, it would require an issignalingf implementation. code size change: -816 bytes. benchmark on x86_64 before, after, speedup: -Os: powf rthruput: 95.14 ns/call 20.04 ns/call 4.75x powf latency: 137.00 ns/call 34.98 ns/call 3.92x -O3: powf rthruput: 92.48 ns/call 13.67 ns/call 6.77x powf latency: 131.11 ns/call 35.15 ns/call 3.73x
2019-04-17math: new exp2f and expfSzabolcs Nagy5-179/+193
from https://github.com/ARM-software/optimized-routines, commit 04884bd04eac4b251da4026900010ea7d8850edc In expf TOINT_INTRINSICS is kept, but is unused, it would require support for __builtin_round and __builtin_lround as single instruction. code size change: +94 bytes. benchmark on x86_64 before, after, speedup: -Os: expf rthruput: 9.19 ns/call 8.11 ns/call 1.13x expf latency: 34.19 ns/call 18.77 ns/call 1.82x exp2f rthruput: 5.59 ns/call 6.52 ns/call 0.86x exp2f latency: 17.93 ns/call 16.70 ns/call 1.07x -O3: expf rthruput: 9.12 ns/call 4.92 ns/call 1.85x expf latency: 34.44 ns/call 18.99 ns/call 1.81x exp2f rthruput: 5.58 ns/call 4.49 ns/call 1.24x exp2f latency: 17.95 ns/call 16.94 ns/call 1.06x
2019-04-17math: new log2fSzabolcs Nagy3-58/+108
from https://github.com/ARM-software/optimized-routines, commit 04884bd04eac4b251da4026900010ea7d8850edc code size change: +177 bytes. benchmark on x86_64 before, after, speedup: -Os: log2f rthruput: 11.38 ns/call 5.99 ns/call 1.9x log2f latency: 35.01 ns/call 22.57 ns/call 1.55x -O3: log2f rthruput: 10.82 ns/call 5.58 ns/call 1.94x log2f latency: 35.13 ns/call 21.04 ns/call 1.67x
2019-04-17math: new logfSzabolcs Nagy3-54/+109
from https://github.com/ARM-software/optimized-routines, commit 04884bd04eac4b251da4026900010ea7d8850edc, with minor changes to better fit into musl. code size change: +289 bytes. benchmark on x86_64 before, after, speedup: -Os: logf rthruput: 8.40 ns/call 6.14 ns/call 1.37x logf latency: 31.79 ns/call 24.33 ns/call 1.31x -O3: logf rthruput: 8.43 ns/call 5.58 ns/call 1.51x logf latency: 32.04 ns/call 20.88 ns/call 1.53x
2019-04-17math: add configuration macrosSzabolcs Nagy1-0/+5
Musl currently aims to support non-nearest rounding mode and does not support SNaNs. These macros allow marking relevant code paths in case these decisions are changed later (they also help documenting the corner cases involved).
2019-04-17math: add macros for static branch prediction hintsSzabolcs Nagy1-0/+9
These don't have an effectw with -Os so not useful with default settings other than documenting the expectation. With --enable-optimize=internal,malloc,string,math the libc.so code size increases by 18K on x86_64 and performance varies in -2% .. +10%.
2019-04-17math: add double precision error handling functionsSzabolcs Nagy6-0/+35
2019-04-17math: add single precision error handling functionsSzabolcs Nagy6-0/+37
These are supposed to be used in tail call positions when handling special cases in new code. (fp exceptions may be raised "naturally" by the common code path if special casing is more effort.) This implements the error handling apis used in https://github.com/ARM-software/optimized-routines without errno setting.
2019-04-17math: add eval_as_float and eval_as_doubleSzabolcs Nagy1-0/+17
Previously type casts or assignments were used for handling excess precision, which assumed standard C99 semantics, but since it's a rarely needed obscure detail, it's better to use explicit helper functions to document where we rely on this. It also helps if the code is used outside of the libc in non-C99 compilation mode: with the default excess precision handling of gcc, explicit inline asm barriers are needed for narrowing on FLT_EVAL_METHOD!=0 targets. I plan to use this in new code with the existing style that uses double_t and float_t as much as possible. One ugliness is that it is required for almost every return statement since that does not drop excess precision (the standard changed this in C11 annex F, but that does not help in non-standard compilation modes or with old compilers).
2019-04-17math: add fp_arch.h with fp_barrier and fp_force_evalSzabolcs Nagy1-6/+65
C99 has ways to support fenv access, but compilers don't implement it and assume nearest rounding mode and no fp status flag access. (gcc has -frounding-math and then it does not assume nearest rounding mode, but it still assumes the compiled code itself does not change the mode. Even if the C99 mechanism was implemented it is not ideal: it requires all code in the library to be compiled with FENV_ACCESS "on" to make it usable in non-nearest rounding mode, but that limits optimizations more than necessary.) The math functions should give reasonable results in all rounding modes (but the quality may be degraded in non-nearest rounding modes) and the fp status flag settings should follow the spec, so fenv side-effects are important and code transformations that break them should be prevented. Unfortunately compilers don't give any help with this, the best we can do is to add fp barriers to the code using volatile local variables (they create a stack frame and undesirable memory accesses to it) or inline asm (gcc specific, requires target specific fp reg constraints, often creates unnecessary reg moves and multiple barriers are needed to express that an operation has side-effects) or extern call (only useful in tail-call position to avoid stack-frame creation and does not work with lto). We assume that in a math function if an operation depends on the input and the output depends on it, then the operation will be evaluated at runtime when the function is called, producing all the expected fenv side-effects (this is not true in case of lto and in case the operation is evaluated with excess precision that is not rounded away). So fp barriers are needed (1) to prevent the move of an operation within a function (in case it may be moved from an unevaluated code path into an evaluated one or if it may be moved across a fenv access), (2) force the evaluation of an operation for its side-effect when it has no input dependency (may be constant folded) or (3) when its output is unused. I belive that fp_barrier and fp_force_eval can take care of these and they should not be needed in hot code paths.
2019-04-17math: remove sun copyright from libm.hSzabolcs Nagy1-23/+0
Nothing is left from the original fdlibm header nor from the bsd modifications to it other than some internal api declarations. Comments are dropped that may be copyrightable content.
2019-04-17math: add asuint, asuint64, asfloat and asdoubleSzabolcs Nagy1-33/+15
Code generation for SET_HIGH_WORD slightly changes, but it only affects pow, otherwise the generated code is unchanged.
2019-04-17math: move complex math out of libm.hSzabolcs Nagy67-80/+87
This makes it easier to build musl math code with a compiler that does not support complex types (tcc) and in general more sensible factorization of the internal headers.
2019-04-10remove external __syscall function and last remaining usersRich Felker18-264/+2
the weak version of __syscall_cp_c was using a tail call to __syscall to avoid duplicating the 6-argument syscall code inline in small static-linked programs, but now that __syscall no longer exists, the inline expansion is no longer duplication. the syscall.h machinery suppported up to 7 syscall arguments, only via an external __syscall function, but we presently have no syscall call points that actually make use of that many, and the kernel only defines 7-argument calling conventions for arm, powerpc (32-bit), and sh. if it turns out we need them in the future, they can easily be added.
2019-04-10overhaul i386 syscall mechanism not to depend on external asm sourceRich Felker7-71/+28
this is the first part of a series of patches intended to make __syscall fully self-contained in the object file produced using syscall.h, which will make it possible for crt1 code to perform syscalls. the (confusingly named) i386 __vsyscall mechanism, which this commit removes, was introduced before the presence of a valid thread pointer was mandatory; back then the thread pointer was setup lazily only if threads were used. the intent was to be able to perform syscalls using the kernel's fast entry point in the VDSO, which can use the sysenter (Intel) or syscall (AMD) instruction instead of int $128, but without inlining an access to the __syscall global at the point of each syscall, which would incur a significant size cost from PIC setup everywhere. the mechanism also shuffled registers/calling convention around to avoid spills of call-saved registers, and to avoid allocating ebx or ebp via asm constraints, since there are plenty of broken-but-supported compiler versions which are incapable of allocating ebx with -fPIC or ebp with -fno-omit-frame-pointer. the new mechanism preserves the properties of avoiding spills and avoiding allocation of ebx/ebp in constraints, but does it inline, using some fairly simple register shuffling, and uses a field of the thread structure rather than global data for the vdso-provided syscall code address. for now, the external __syscall function is refactored not to use the old __vsyscall so it can be kept, but the intent is to remove it too.
2019-04-09in membarrier fallback, allow for possibility that sigaction failsRich Felker1-8/+9
this is a workaround to avoid a crashing regression on qemu-user when dynamic TLS is installed at dlopen time. the sigaction syscall should not be able to fail, but it does fail for implementation-internal signals under qemu user-level emulation if the host libc qemu is running under reserves the same signals for implementation-internal use, since qemu makes no provision to redirect/emulate them. after sigaction fails, the subsequent tkill would terminate the process abnormally as the default action. no provision to account for membarrier failing is made in the dynamic linker code that installs new TLS. at the formal level, the missing barrier in this case is incorrect, and perhaps we should fail the dlopen operation, but in practice all the archs we support (and probably all real-world archs except alpha, which isn't yet supported) should give the right behavior with no barrier at all as a consequence of consume-order properties. in the long term, this workaround should be supplemented or replaced by something better -- a different fallback approach to ensuring memory consistency, or dynamic allocation of implementation-internal signals. the latter is appealing in that it would allow cancellation to work under qemu-user too, and would even allow many levels of nested emulation.
2019-04-03fix unintended global symbols in atanl.cDan Gohman1-3/+3
Mark atanhi, atanlo, and aT in atanl.c as static, as they're not intended to be part of the public API. These are already static in the LDBL_MANT_DIG == 64 code, so this patch is just making the LDBL_MANT_DIG == 113 code do the same thing.
2019-04-02use __strchrnul instead of strchr and strlen in execvpeFrediano Ziglio1-2/+1
The result is the same but takes less code. Note that __execvpe calls getenv which calls __strchrnul so even using static output the size of the executable won't grow.
2019-04-01fix harmless-by-chance typo in priority inheritance mutex codeRich Felker1-1/+1
commit 54ca677983d47529bab8752315ac1a2b49888870 inadvertently introduced bitwise and where logical and was intended. since the right-hand operand is always 0 or -1 whenever the left-hand operand is nonzero, the behavior happened to be equivalent.
2019-03-31implement priority inheritance mutexesRich Felker4-8/+93
priority inheritance is a feature to mitigate priority inversion situations, where a execution of a medium-priority thread can unboundedly block forward progress of a high-priority thread when a lock it needs is held by a low-priority thread. the natural way to do priority inheritance would be with a simple futex flag to donate the calling thread's priority to a target thread while it waits on the futex. unfortunately, linux does not offer such an interface, but instead insists on implementing the whole locking protocol in kernelspace with special futex commands that exist solely for the purpose of doing PI mutexes. this would require the entire "trylock" logic to be duplicated in the timedlock code path for PI mutexes, since, once the previous lock holder releases the lock and the futex call returns, the lock is already held by the caller. obviously such code duplication is undesirable. instead, I've made the PI timedlock success path set the mutex lock count to -1, which can be thought of as "not yet complete", since a lock count of 0 is "locked, with no recursive references". a simple branch in a non-hot path of pthread_mutex_trylock can then see and act on this state, skipping past the code that would check and take the lock to the same code path that runs after the lock is obtained for a non-PI mutex. because we're forced to let the kernel perform the actual lock and unlock operations whenever the mutex is contended, we have to patch things up when it does the wrong thing: 1. the lock operation is not aware of whether the mutex is error-checking, so it will always fail with EDEADLK rather than deadlocking. 2. the lock operation is not aware of whether the mutex is robust, so it will successfully obtain mutexes in the owner-died state even if they're non-robust, whereas this operation should deadlock. 3. the unlock operation always sets the lock value to zero, whereas for robust mutexes, we want to set it to a special value indicating that the mutex obtained after its owner died was unlocked without marking it consistent, so that future operations all fail with ENOTRECOVERABLE. the first of these is easy to solve, just by performing a futex wait on a dummy futex address to simulate deadlock or ETIMEDOUT as appropriate. but problems 2 and 3 interact in a nasty way. to solve problem 2, we need to back out the spurious success. but if waiters are present -- which we can't just ignore, because even if we don't want to wake them, the calling thread is incorrectly inheriting their priorities -- this requires using the kernel's unlock operation, which will zero the lock value, thereby losing the "owner died with lock held" state. to solve these problems, we overload the mutex's waiters field, which is unused for PI mutexes since they don't call the normal futex wait functions, as an indicator that the PI mutex is permanently non-lockable. originally I wanted to use the count field, but there is one code path that needs to access this flag without synchronization: trylock's CAS failure path needs to be able to decide whether to fail with EBUSY or ENOTRECOVERABLE, the waiters field is already treated as a relaxed-order atomic in our memory model, so this works out nicely.
2019-03-29clean up access to mutex type in pthread_mutex_trylockRich Felker1-2/+2
there was no point in masking off the pshared bit when first loading the type, since every subsequent access involves a mask anyway. not masking it may avoid a subsequent load to check the pshared flag, and it's just simpler.
generated by cgit v1.2.3-70-g09d2 (git 2.46.0) at 2024-11-06 07:25:14 +0000