|author||Rich Felker <firstname.lastname@example.org>||2020-05-21 23:32:45 -0400|
|committer||Rich Felker <email@example.com>||2020-05-22 17:39:57 -0400|
don't use libc.threads_minus_1 as relaxed atomic for skipping locks
after all but the last thread exits, the next thread to observe libc.threads_minus_1==0 and conclude that it can skip locking fails to synchronize with any changes to memory that were made by the last-exiting thread. this can produce data races. on some archs, at least x86, memory synchronization is unlikely to be a problem; however, with the inline locks in malloc, skipping the lock also eliminated the compiler barrier, and caused code that needed to re-check chunk in-use bits after obtaining the lock to reuse a stale value, possibly from before the process became single-threaded. this in turn produced corruption of the heap state. some uses of libc.threads_minus_1 remain, especially for allocation of new TLS in the dynamic linker; otherwise, it could be removed entirely. it's made non-volatile to reflect that the remaining accesses are only made under lock on the thread list. instead of libc.threads_minus_1, libc.threaded is now used for skipping locks. the difference is that libc.threaded is permanently true once an additional thread has been created. this will produce some performance regression in processes that are mostly single-threaded but occasionally creating threads. in the future it may be possible to bring back the full lock-skipping, but more care needs to be taken to produce a safe design.
Diffstat (limited to 'src/string/memchr.c')
0 files changed, 0 insertions, 0 deletions