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Revert changes to convert to ->poll_mask() and aio IOCB_CMD_POLL

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The poll() changes were not well thought out, and completely
unexplained.  They also caused a huge performance regression, because
"->poll()" was no longer a trivial file operation that just called down
to the underlying file operations, but instead did at least two indirect
calls.

Indirect calls are sadly slow now with the Spectre mitigation, but the
performance problem could at least be largely mitigated by changing the
"->get_poll_head()" operation to just have a per-file-descriptor pointer
to the poll head instead.  That gets rid of one of the new indirections.

But that doesn't fix the new complexity that is completely unwarranted
for the regular case.  The (undocumented) reason for the poll() changes
was some alleged AIO poll race fixing, but we don't make the common case
slower and more complex for some uncommon special case, so this all
really needs way more explanations and most likely a fundamental
redesign.

[ This revert is a revert of about 30 different commits, not reverted
  individually because that would just be unnecessarily messy  - Linus ]

Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Christoph Hellwig <hch@lst.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Linus Torvalds 2018-06-28 09:43:44 -07:00
parent f57494321c
commit a11e1d432b
80 changed files with 302 additions and 451 deletions
Download patch file Download diff file Expand all files Collapse all files

19
fs/eventfd.c
View file

@ -101,20 +101,14 @@ static int eventfd_release(struct inode *inode, struct file *file)
return 0;
}
static struct wait_queue_head *
eventfd_get_poll_head(struct file *file, __poll_t events)
{
struct eventfd_ctx *ctx = file->private_data;
return &ctx->wqh;
}
static __poll_t eventfd_poll_mask(struct file *file, __poll_t eventmask)
static __poll_t eventfd_poll(struct file *file, poll_table *wait)
{
struct eventfd_ctx *ctx = file->private_data;
__poll_t events = 0;
u64 count;
poll_wait(file, &ctx->wqh, wait);
/*
* All writes to ctx->count occur within ctx->wqh.lock. This read
* can be done outside ctx->wqh.lock because we know that poll_wait
@ -156,11 +150,11 @@ static __poll_t eventfd_poll_mask(struct file *file, __poll_t eventmask)
count = READ_ONCE(ctx->count);
if (count > 0)
events |= (EPOLLIN & eventmask);
events |= EPOLLIN;
if (count == ULLONG_MAX)
events |= EPOLLERR;
if (ULLONG_MAX - 1 > count)
events |= (EPOLLOUT & eventmask);
events |= EPOLLOUT;
return events;
}
@ -311,8 +305,7 @@ static const struct file_operations eventfd_fops = {
.show_fdinfo = eventfd_show_fdinfo,
#endif
.release = eventfd_release,
.get_poll_head = eventfd_get_poll_head,
.poll_mask = eventfd_poll_mask,
.poll = eventfd_poll,
.read = eventfd_read,
.write = eventfd_write,
.llseek = noop_llseek,