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aio: fix race on file length (workaround) 

It appears that concurrent reads and writes on AIO seem to
result in inconsistent reads in some very rare cases, due to
races. Sometimes, the inode claims that the file has been already
appended by a write operation, but the data has not actually hit
the page cache, such that a concurrent read gets NULL blocks.

This is a workaround by "misusing" the mapfree infrastructure.
It depends on uniqueness of file names, which is not the right thing
to do.

Best fix would be in the kernel.
This commit is contained in:
Thomas Schoebel-Theuer 2013-04-16 09:42:34 +02:00
parent fa48bb94d0
commit 814b92ba19
1 changed files with 15 additions and 0 deletions
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15
kernel/mars_aio.c
View File

@ -630,6 +630,8 @@ static int aio_event_thread(void *data)
MARS_IO("AIO done %p pos = %lld len = %d rw = %d\n", mref, mref->ref_pos, mref->ref_len, mref->ref_rw);
mapfree_set(output->mf, mref->ref_pos, mref->ref_pos + mref->ref_len);
if (output->brick->o_fdsync
&& err >= 0
&& mref->ref_rw != READ
@ -879,6 +881,19 @@ static int aio_get_info(struct aio_output *output, struct mars_info *info)
info->current_size = i_size_read(file->f_mapping->host);
MARS_DBG("determined file size = %lld\n", info->current_size);
/* Workaround for races in the page cache.
*
* It appears that concurrent reads and writes seem to
* result in inconsistent reads in some very rare cases, due to
* races. Sometimes, the inode claims that the file has been already
* appended by a write operation, but the data has not actually hit
* the page cache, such that a concurrent read gets NULL blocks.
*/
if (output->mf->mf_max >= 0 && output->mf->mf_max < info->current_size) {
MARS_DBG("correcting file length from %lld to %lld\n", info->current_size, output->mf->mf_max);
info->current_size = output->mf->mf_max;
}
info->backing_file = file;
return 0;
}