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| -rw-r--r-- | ChangeLog | 5 | ||||
| -rw-r--r-- | manual/llio.texi | 25 |
2 files changed, 30 insertions, 0 deletions
diff --git a/ChangeLog b/ChangeLog index 6874e5404d..a28bb3b345 100644 --- a/ChangeLog +++ b/ChangeLog @@ -1,3 +1,8 @@ +2014-10-29 Carlos O'Donell <carlos@redhat.com> + + * manual/llio.texi: Add comments discussing why write() may be + considered MT-unsafe on Linux. + 2014-10-28 Carlos O'Donell <carlos@redhat.com> * dl-load.c (local_strdup): Remove. diff --git a/manual/llio.texi b/manual/llio.texi index 864060dc71..393ddf30ab 100644 --- a/manual/llio.texi +++ b/manual/llio.texi @@ -466,6 +466,31 @@ When the source file is compiled with @code{_FILE_OFFSET_BITS == 64} on a @comment POSIX.1 @deftypefun ssize_t write (int @var{filedes}, const void *@var{buffer}, size_t @var{size}) @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}} +@c Some say write is thread-unsafe on Linux without O_APPEND. In the VFS layer +@c the vfs_write() does no locking around the acquisition of a file offset and +@c therefore multiple threads / kernel tasks may race and get the same offset +@c resulting in data loss. +@c +@c See: +@c http://thread.gmane.org/gmane.linux.kernel/397980 +@c http://lwn.net/Articles/180387/ +@c +@c The counter argument is that POSIX only says that the write starts at the +@c file position and that the file position is updated *before* the function +@c returns. What that really means is that any expectation of atomic writes is +@c strictly an invention of the interpretation of the reader. Data loss could +@c happen if two threads start the write at the same time. Only writes that +@c come after the return of another write are guaranteed to follow the other +@c write. +@c +@c The other side of the coin is that POSIX goes on further to say in +@c "2.9.7 Thread Interactions with Regular File Operations" that threads +@c should never see interleaving sets of file operations, but it is insane +@c to do anything like that because it kills performance, so you don't get +@c those guarantees in Linux. +@c +@c So we mark it thread safe, it doesn't blow up, but you might loose +@c data, and we don't strictly meet the POSIX requirements. The @code{write} function writes up to @var{size} bytes from @var{buffer} to the file with descriptor @var{filedes}. The data in @var{buffer} is not necessarily a character string and a null character is |