The inode chunk allocation path can lead to deadlock conditions if a transaction is dirtied with an AGF (to fix up the freelist) for an AG that cannot satisfy the actual allocation request. This code path is written to try and avoid this scenario, but it can be reproduced by running xfstests generic/270 in a loop on a 512b fs. An example situation is: - process A attempts an inode allocation on AG 3, modifies the freelist, fails the allocation and ultimately moves on to AG 0 with the AG 3 AGF held - process B is doing a free space operation (i.e., truncate) and acquires the AG 0 AGF, waits on the AG 3 AGF - process A acquires the AG 0 AGI, waits on the AG 0 AGF (deadlock) The problem here is that process A acquired the AG 3 AGF while moving on to AG 0 (and releasing the AG 3 AGI with the AG 3 AGF held). xfs_dialloc() makes one pass through each of the AGs when attempting to allocate an inode chunk. The expectation is a clean transaction if a particular AG cannot satisfy the allocation request. xfs_ialloc_ag_alloc() is written to support this through use of the minalignslop allocation args field. When using the agi->agi_newino optimization, we attempt an exact bno allocation request based on the location of the previously allocated chunk. minalignslop is set to inform the allocator that we will require alignment on this chunk, and thus to not allow the request for this AG if the extra space is not available. Suppose that the AG in question has just enough space for this request, but not at the requested bno. xfs_alloc_fix_freelist() will proceed as normal as it determines the request should succeed, and thus it is allowed to modify the agf. xfs_alloc_ag_vextent() ultimately fails because the requested bno is not available. In response, the caller moves on to a NEAR_BNO allocation request for the same AG. The alignment is set, but the minalignslop field is never reset. This increases the overall requirement of the request from the first attempt. If this delta is the difference between allocation success and failure for the AG, xfs_alloc_fix_freelist() rejects this request outright the second time around and causes the allocation request to unnecessarily fail for this AG. To address this situation, reset the minalignslop field immediately after use and prevent it from leaking into subsequent requests. Signed-off-by: Brian Foster <bfoster@xxxxxxxxxx> --- v2: - Reset minalignslop immediately after use rather than prior to the subsequent request and add a comment. [dchinner] fs/xfs/xfs_ialloc.c | 12 ++++++++++++ 1 file changed, 12 insertions(+) diff --git a/fs/xfs/xfs_ialloc.c b/fs/xfs/xfs_ialloc.c index 5d7f105..a57843f 100644 --- a/fs/xfs/xfs_ialloc.c +++ b/fs/xfs/xfs_ialloc.c @@ -363,6 +363,18 @@ xfs_ialloc_ag_alloc( args.minleft = args.mp->m_in_maxlevels - 1; if ((error = xfs_alloc_vextent(&args))) return error; + + /* + * This request might have dirtied the transaction if the AG can + * satisfy the request, but the exact block was not available. + * If the allocation did fail, subsequent requests will relax + * the exact agbno requirement and increase the alignment + * instead. It is critical that the total size of the request + * (len + alignment + slop) does not increase from this point + * on, so reset minalignslop to ensure it is not included in + * subsequent requests. + */ + args.minalignslop = 0; } else args.fsbno = NULLFSBLOCK; -- 1.8.3.1 _______________________________________________ xfs mailing list xfs@xxxxxxxxxxx http://oss.sgi.com/mailman/listinfo/xfs
