[patch] epoll avoid double-inserts in case of EFAULT

From: Davide Libenzi
Date: Fri Oct 17 2008 - 19:17:56 EST

First Episode:

Thomas found that there is an unnecessary (always true) test in
ep_send_events(). The callback never inserts into ->rdllink while the
send loop is performed, and also does the ~EP_PRIVATE_BITS test. Given
we're holding the mutex during this time, the conditions tested inside the
loop are always true.
This patch drops the test done inside the re-insertion loop.

Second Episode:

The test "!ep_is_linked(&epi->rdllink)" wasn't there because we inserts
into ->rdllink, but because the send-events loop might terminate before
the whole list is scanned (-EFAULT). In such cases, when the loop
terminates early, and when a (leftover) file received an event while we're
performing the lockless loop, we need such test to avoid to double insert
the epoll items. The list_splice() done a few steps below, will correctly
re-insert the ones that were left on "txlist".
This should be the reason for the thread "2.6.27-05178-g2e532d6: list_add
corruption" from Alexander Beregalov.

Signed-off-by: Davide Libenzi <davidel@xxxxxxxxxxxxxxx>

- Davide

fs/eventpoll.c | 11 +++++++++--
1 file changed, 9 insertions(+), 2 deletions(-)

Index: linux-2.6.mod/fs/eventpoll.c
--- linux-2.6.mod.orig/fs/eventpoll.c 2008-10-17 15:51:09.000000000 -0700
+++ linux-2.6.mod/fs/eventpoll.c 2008-10-17 15:54:14.000000000 -0700
@@ -930,8 +930,15 @@
* inside the main ready-list here.
for (nepi = ep->ovflist; (epi = nepi) != NULL;
- nepi = epi->next, epi->next = EP_UNACTIVE_PTR)
- list_add_tail(&epi->rdllink, &ep->rdllist);
+ nepi = epi->next, epi->next = EP_UNACTIVE_PTR) {
+ /*
+ * If the above loop quit with errors, the epoll item might still
+ * be linked to "txlist", and the list_splice() done below will
+ * take care of those cases.
+ */
+ if (!ep_is_linked(&epi->rdllink))
+ list_add_tail(&epi->rdllink, &ep->rdllist);
+ }
* We need to set back ep->ovflist to EP_UNACTIVE_PTR, so that after
* releasing the lock, events will be queued in the normal way inside

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