Messages in this thread |  | | | Date | Tue, 28 Jun 2011 07:52:06 -0700 | | From | Darren Hart <> | | Subject | Re: [PATCH v3] futex: Fix regression with read only mappings |
| |
On 06/28/2011 03:54 AM, Peter Zijlstra wrote: > On Mon, 2011-06-27 at 17:22 -0500, Shawn Bohrer wrote: >> commit 7485d0d3758e8e6491a5c9468114e74dc050785d (futexes: Remove rw >> parameter from get_futex_key()) in 2.6.33 introduced a user-mode >> regression in that it additionally prevented futex operations on a >> region within a read only memory mapped file. For example this breaks >> workloads that have one or more reader processes doing a FUTEX_WAIT on a >> futex within a read only shared mapping, and a writer processes that has >> a writable mapping issuing the FUTEX_WAKE. >> >> This fixes the regression for futex operations that should be valid on >> RO mappings by trying a RO get_user_pages_fast() when the RW >> get_user_pages_fast() fails so as not to slow down the common path of >> writable anonymous maps and bailing when we used the RO path on >> anonymous memory. >> >> While fixing the regression this patch opens up two possible bad >> scenarios as identified by KOSAKI Motohiro: >> >> 1) This patch also allows FUTEX_WAIT on RO private mappings which have >> the following corner case. >> >> Thread-A: call futex(FUTEX_WAIT, memory-region-A). >> get_futex_key() return inode based key. >> sleep on the key >> Thread-B: call mprotect(PROT_READ|PROT_WRITE, memory-region-A) >> Thread-B: write memory-region-A. >> COW happen. This process's memory-region-A become related >> to new COWed private (ie PageAnon=1) page. >> Thread-B: call futex(FUETX_WAKE, memory-region-A). >> get_futex_key() return mm based key. >> IOW, we fail to wake up Thread-A. >> >> 2) Current futex code doesn't handle zero page properly. >> >> Read mode get_user_pages() can return zero page, but current futex >> code doesn't handle it at all. Then, zero page makes infinite loop >> internally. >> >> This Patch is based on Peter Zijlstra's initial patch with modifications to >> only allow RO mappings for futex operations that need VERIFY_READ access. >> >> Reported-by: David Oliver <david@rgmadvisors.com> >> Signed-off-by: Shawn Bohrer <sbohrer@rgmadvisors.com> > > Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
I think we need to address #2 above first.
1) Is the loop killable? 2) If not, and we try to catch zero_page scenario, we need to ensure RO sparse file mapping work. Peter notes that these are probably fine, filemap.c doesn't seem to use the zero page - but we need to test.
It's agreed, I believe, between myself, Peter, and Shawn that we will document Private RW mappings as unsupported.
-- Darren Hart Intel Open Source Technology Center Yocto Project - Linux Kernel
|  |