linux-kernel - [patch] shared tag queue barrier comment

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [thread-next>] [day] [month] [year] [list]

Message-ID: <20070914064112.GA25277@wotan.suse.de>
Date:	Fri, 14 Sep 2007 08:41:12 +0200
From:	Nick Piggin <npiggin@...e.de>
To:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Jens Axboe <axboe@...nel.dk>
Subject: [patch] shared tag queue barrier comment


Should add some comments for the tag barriers (they won't be so important
if we can switch over to the explicit _lock bitops, but for now we should
make it clear).

Jens' original patch said a barrier after the test_and_clear_bit was also
required. I can't see why (and it would prevent the use of the _lock bitop).

--
Index: linux-2.6/block/ll_rw_blk.c
===================================================================
--- linux-2.6.orig/block/ll_rw_blk.c
+++ linux-2.6/block/ll_rw_blk.c
@@ -1085,6 +1085,12 @@ void blk_queue_end_tag(struct request_qu
 
 	bqt->tag_index[tag] = NULL;
 
+	/*
+	 * We use test_and_clear_bit's memory ordering properties here.
+	 * The tag_map bit acts as a lock for tag_index[bit], so we need
+	 * a barrer before clearing the bit (precisely: release semantics).
+	 * Could use clear_bit_unlock when it is merged.
+	 */
 	if (unlikely(!test_and_clear_bit(tag, bqt->tag_map))) {
 		printk(KERN_ERR "%s: attempt to clear non-busy tag (%d)\n",
 		       __FUNCTION__, tag);
@@ -1137,6 +1143,10 @@ int blk_queue_start_tag(struct request_q
 			return 1;
 
 	} while (test_and_set_bit(tag, bqt->tag_map));
+	/*
+	 * We rely on test_and_set_bit providing lock memory ordering semantics
+	 * (could use test_and_set_bit_lock when it is merged).
+	 */
 
 	rq->cmd_flags |= REQ_QUEUED;
 	rq->tag = tag;
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/