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From: Michael Haggerty <mhagger@alum.mit.edu>
To: Junio C Hamano <gitster@pobox.com>
Cc: "Nguyễn Thái Ngọc Duy" <pclouds@gmail.com>,
	"Stefan Beller" <sbeller@google.com>, "Jeff King" <peff@peff.net>,
	"Ævar Arnfjörð Bjarmason" <avarab@gmail.com>,
	"David Turner" <novalis@novalis.org>,
	git@vger.kernel.org, "Michael Haggerty" <mhagger@alum.mit.edu>
Subject: [PATCH 17/23] get_packed_ref_cache(): assume "packed-refs" won't change while locked
Date: Wed, 17 May 2017 14:05:40 +0200	[thread overview]
Message-ID: <eedc8f85c8237515bc66584649b1e3f3d547049c.1495014840.git.mhagger@alum.mit.edu> (raw)
In-Reply-To: <cover.1495014840.git.mhagger@alum.mit.edu>

If we've got the "packed-refs" file locked, then it can't change;
there's no need to keep calling `stat_validity_check()` on it.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
---
 refs/files-backend.c | 16 +++++++++++-----
 1 file changed, 11 insertions(+), 5 deletions(-)

diff --git a/refs/files-backend.c b/refs/files-backend.c
index c549a8d956..a910c44f38 100644
--- a/refs/files-backend.c
+++ b/refs/files-backend.c
@@ -354,13 +354,18 @@ static void files_ref_path(struct files_ref_store *refs,
 
 /*
  * Get the packed_ref_cache for the specified files_ref_store,
- * creating it if necessary.
+ * creating and populating it if it hasn't been read before or if the
+ * file has been changed (according to its `validity` field) since it
+ * was last read. On the other hand, if we hold the lock, then assume
+ * that the file hasn't been changed out from under us, so skip the
+ * extra `stat()` call in `stat_validity_check()`.
  */
 static struct packed_ref_cache *get_packed_ref_cache(struct files_ref_store *refs)
 {
 	const char *packed_refs_file = files_packed_refs_path(refs);
 
 	if (refs->packed &&
+	    !is_lock_file_locked(&refs->packlock) &&
 	    !stat_validity_check(&refs->packed->validity, packed_refs_file))
 		clear_packed_ref_cache(refs);
 
@@ -1300,10 +1305,11 @@ static int lock_packed_refs(struct files_ref_store *refs, int flags)
 			    flags, timeout_value) < 0)
 		return -1;
 	/*
-	 * Get the current packed-refs while holding the lock.  If the
-	 * packed-refs file has been modified since we last read it,
-	 * this will automatically invalidate the cache and re-read
-	 * the packed-refs file.
+	 * Get the current packed-refs while holding the lock. It is
+	 * important that we call `get_packed_ref_cache()` before
+	 * setting `packed_ref_cache->lock`, because otherwise the
+	 * former will see that the file is locked and assume that the
+	 * cache can't be stale.
 	 */
 	packed_ref_cache = get_packed_ref_cache(refs);
 	/* Increment the reference count to prevent it from being freed: */
-- 
2.11.0


  parent reply	other threads:[~2017-05-17 12:06 UTC|newest]

Thread overview: 73+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2017-05-17 12:05 [PATCH 00/23] Prepare to separate out a packed_ref_store Michael Haggerty
2017-05-17 12:05 ` [PATCH 01/23] t3600: clean up permissions test properly Michael Haggerty
2017-05-17 12:42   ` Jeff King
2017-05-17 14:01     ` Michael Haggerty
2017-05-18  4:10   ` Junio C Hamano
2017-05-19  3:37     ` Michael Haggerty
2017-05-17 12:05 ` [PATCH 02/23] refs.h: clarify docstring for the ref_transaction_update()-related fns Michael Haggerty
2017-05-17 16:46   ` Stefan Beller
2017-05-18  4:13     ` Junio C Hamano
2017-05-17 12:05 ` [PATCH 03/23] ref_iterator_begin_fn(): fix docstring Michael Haggerty
2017-05-17 12:05 ` [PATCH 04/23] prefix_ref_iterator: don't trim too much Michael Haggerty
2017-05-17 12:55   ` Jeff King
2017-05-17 14:11     ` Michael Haggerty
2017-05-17 14:22       ` Jeff King
2017-05-18  4:19   ` Junio C Hamano
2017-05-18  4:50     ` Michael Haggerty
2017-05-17 12:05 ` [PATCH 05/23] refs_ref_iterator_begin(): don't check prefixes redundantly Michael Haggerty
2017-05-17 12:59   ` Jeff King
2017-05-17 14:21     ` Michael Haggerty
2017-05-17 12:05 ` [PATCH 06/23] refs: use `size_t` indexes when iterating over ref transaction updates Michael Haggerty
2017-05-17 16:59   ` Stefan Beller
2017-05-18  4:55     ` Michael Haggerty
2017-05-17 12:05 ` [PATCH 07/23] ref_store: take `logmsg` parameter when deleting references Michael Haggerty
2017-05-17 13:12   ` Jeff King
2017-05-17 15:01     ` Michael Haggerty
2017-05-17 15:03       ` Jeff King
2017-05-17 12:05 ` [PATCH 08/23] lockfile: add a new method, is_lock_file_locked() Michael Haggerty
2017-05-17 13:12   ` Jeff King
2017-05-17 12:05 ` [PATCH 09/23] files-backend: move `lock` member to `files_ref_store` Michael Haggerty
2017-05-17 13:15   ` Jeff King
2017-05-17 15:49     ` Michael Haggerty
2017-05-17 12:05 ` [PATCH 10/23] files_ref_store: put the packed files lock directly in this struct Michael Haggerty
2017-05-17 13:17   ` Jeff King
2017-05-17 15:05     ` Michael Haggerty
2017-05-17 17:18     ` Stefan Beller
2017-05-18  0:18       ` Brandon Williams
2017-05-19  4:00       ` Michael Haggerty
2017-05-18  0:17     ` Brandon Williams
2017-05-18  1:11       ` Jeff King
2017-05-18 15:42         ` Brandon Williams
2017-05-17 12:05 ` [PATCH 11/23] files_transaction_cleanup(): new helper function Michael Haggerty
2017-05-17 13:19   ` Jeff King
2017-05-19  4:49     ` Michael Haggerty
2017-05-17 17:26   ` Stefan Beller
2017-05-19  4:42     ` Michael Haggerty
2017-05-17 12:05 ` [PATCH 12/23] ref_transaction_commit(): break into multiple functions Michael Haggerty
2017-05-17 17:44   ` Stefan Beller
2017-05-19  7:58     ` Michael Haggerty
2017-05-17 12:05 ` [PATCH 13/23] ref_update_reject_duplicates(): expose function to whole refs module Michael Haggerty
2017-05-17 12:05 ` [PATCH 14/23] ref_update_reject_duplicates(): use `size_t` rather than `int` Michael Haggerty
2017-05-17 12:05 ` [PATCH 15/23] ref_update_reject_duplicates(): add a sanity check Michael Haggerty
2017-05-17 12:05 ` [PATCH 16/23] should_pack_ref(): new function, extracted from `files_pack_refs()` Michael Haggerty
2017-05-17 12:05 ` Michael Haggerty [this message]
2017-05-17 17:57   ` [PATCH 17/23] get_packed_ref_cache(): assume "packed-refs" won't change while locked Stefan Beller
2017-05-18  1:15     ` Jeff King
2017-05-18 16:58       ` Stefan Beller
2017-05-17 12:05 ` [PATCH 18/23] read_packed_refs(): do more of the work of reading packed refs Michael Haggerty
2017-05-17 12:05 ` [PATCH 19/23] read_packed_refs(): report unexpected fopen() failures Michael Haggerty
2017-05-17 13:28   ` Jeff King
2017-05-17 15:27     ` Michael Haggerty
2017-05-18  4:57     ` Junio C Hamano
2017-05-18  5:08       ` Jeff King
2017-05-17 12:05 ` [PATCH 20/23] refs_ref_iterator_begin(): handle `GIT_REF_PARANOIA` Michael Haggerty
2017-05-17 13:29   ` Jeff King
2017-05-17 12:05 ` [PATCH 21/23] create_ref_entry(): remove `check_name` option Michael Haggerty
2017-05-17 12:05 ` [PATCH 22/23] ref-filter: limit traversal to prefix Michael Haggerty
2017-05-17 13:38   ` Jeff King
2017-05-19 10:02     ` Michael Haggerty
2017-05-17 12:05 ` [PATCH 23/23] cache_ref_iterator_begin(): avoid priming unneeded directories Michael Haggerty
2017-05-17 13:42 ` [PATCH 00/23] Prepare to separate out a packed_ref_store Jeff King
2017-05-17 18:14   ` Stefan Beller
2017-05-18 17:14 ` Johannes Sixt
2017-05-18 17:22   ` Jeff King

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