On 2014-05-02 17:05, Peter Hurley wrote: > On 05/02/2014 10:56 AM, Peter Hurley wrote: >> Commit 6a20dbd6caa2358716136144bf524331d70b1e03, >> "tty: Fix race condition between __tty_buffer_request_room and >> flush_to_ldisc" >> correctly identifies an unsafe race condition between >> __tty_buffer_request_room() and flush_to_ldisc(), where the consumer >> flush_to_ldisc() prematurely advances the head before consuming the >> last of the data committed. For example: >> >> CPU 0 | CPU 1 >> __tty_buffer_request_room | flush_to_ldisc >> ... | ... >> | count = head->commit - head->read >> n = tty_buffer_alloc() | >> b->commit = b->used | >> b->next = n | >> | if (!count) /* T */ >> | if (head->next == NULL) /* F */ >> | buf->head = head->next >> >> In this case, buf->head has been advanced but head->commit may have >> been updated with a new value. >> >> Instead of reintroducing an unnecessary lock, fix the race locklessly. >> Read the commit-next pair in the reverse order of writing, which guarantees >> the commit value read is the latest value written if the head is >> advancing.
This is a fine solution! I'll verify this against my previous experimental setup (3.12.x and 3.12.x-rt25), but I dont't expect any problems. >> >> Reported-by: Manfred Schlaegl <[email protected]> >> Cc: <[email protected]> # 3.12.x+ > > The patch submitted by Manfred notes the commits which introduced the > race [1], but attributes those commits to the 3.11 cycle. Those commits > were merged in the 3.12 cycle. You are right. I'm sorry for this. Regars, Manfred -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
