Amerigo Wang <amwang@xxxxxxxxxx> wrote:
rwsem_is_locked() tests ->activity without locks, so we should always
keep ->activity consistent. However, the code in __rwsem_do_wake()
breaks this rule, it updates ->activity after _all_ readers waken up,
this may give some reader a wrong ->activity value, thus cause
rwsem_is_locked() behaves wrong.
Quote from Andrew:
"
- we have one or more processes sleeping in down_read(), waiting for access.
- we wake one or more processes up without altering ->activity
- they start to run and they do rwsem_is_locked(). This incorrectly
returns "false", because the waker process is still crunching away in
__rwsem_do_wake().
- the waker now alters ->activity, but it was too late.
"
So we need get a spinlock to protect this. And rwsem_is_locked()
should not block, thus we use spin_trylock.
Reported-by: Brian Behlendorf <behlendorf1@xxxxxxxx>
Cc: Ben Woodard <bwoodard@xxxxxxxx>
Cc: David Howells <dhowells@xxxxxxxxxx>
Signed-off-by: WANG Cong <amwang@xxxxxxxxxx>
I'd say the comment in __rwsem_do_wake() is unnecessary, but other than
that...
Acked-by: David Howells <dhowells@xxxxxxxxxx>