From b7af3a8c36426811762bf331e3938f9d67b7429e Mon Sep 17 00:00:00 2001
From: Simon Rettberg
Date: Fri, 2 Aug 2019 16:58:34 +0200
Subject: [SERVER] Make image->users atomic and get rid of some locking

With this change it should be safe to read the users count of an image
without locking first, assuming you already have a reference on the
image or are otherwise sure it cannot be freed, i.e. in an active
uplink. Updating users, or checking whether it's 0 in order to free the
image should only be done while holding the imageListLock.
---
 src/server/globals.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

(limited to 'src/server/globals.h')

diff --git a/src/server/globals.h b/src/server/globals.h
index b248800..73eb563 100644
--- a/src/server/globals.h
+++ b/src/server/globals.h
@@ -117,7 +117,7 @@ struct _dnbd3_image
 	uint32_t masterCrc32;  // CRC-32 of the crc-32 list
 	int readFd;            // used to read the image. Used from multiple threads, so use atomic operations (pread et al)
 	int completenessEstimate; // Completeness estimate in percent
-	int users;             // clients currently using this image
+	atomic_int users;      // clients currently using this image. XXX Lock on imageListLock when modifying and checking whether the image should be freed. Reading it elsewhere is fine without the lock.
 	int id;                // Unique ID of this image. Only unique in the context of this running instance of DNBD3-Server
 	bool working;          // true if image exists and completeness is == 100% or a working upstream proxy is connected
 	uint16_t rid;          // revision of image
-- 
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