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authorRich Felker <dalias@aerifal.cx>2016-06-27 15:18:13 -0400
committerRich Felker <dalias@aerifal.cx>2016-06-27 15:18:13 -0400
commit384d103d94dba0472a587861f67d7ed6e8955f86 (patch)
treeb113865f16b998f0c8d3d7fe19f33c39785a5e7b /src/network/send.c
parent6cec7bc57f599f43f4041cec2093e3c9231dbaab (diff)
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fix failure to obtain EOWNERDEAD status for process-shared robust mutexes
Linux's documentation (robust-futex-ABI.txt) claims that, when a
process dies with a futex on the robust list, bit 30 (0x40000000) is
set to indicate the status. however, what actually happens is that
bits 0-30 are replaced with the value 0x40000000, i.e. bits 0-29
(containing the old owner tid) are cleared at the same time bit 30 is
set.

our userspace-side code for robust mutexes was written based on that
documentation, assuming that kernel would never produce a futex value
of 0x40000000, since the low (owner) bits would always be non-zero.
commit d338b506e39b1e2c68366b12be90704c635602ce introduced this
assumption explicitly while fixing another bug in how non-recoverable
status for robust mutexes was tracked. presumably the tests conducted
at that time only checked non-process-shared robust mutexes, which are
handled in pthread_exit (which implemented the documented kernel
protocol, not the actual one) rather than by the kernel.

change pthread_exit robust list processing to match the kernel
behavior, clearing bits 0-29 while setting bit 30, and use the value
0x7fffffff instead of 0x40000000 to encode non-recoverable status. the
choice of value here is arbitrary; any value with at least one of bits
0-29 set should work just as well,
Diffstat (limited to 'src/network/send.c')
0 files changed, 0 insertions, 0 deletions