| > | > +static int ccid_request_module(u8 id)
| > | > +{
| > | > + if (!in_atomic()) {
| > | > + ccids_read_lock();
| > | > + if (ccids[id] == NULL) {
| > | > + ccids_read_unlock();
| > | > + return request_module("net-dccp-ccid-%d", id);
| > | > + }
| > | > + ccids_read_unlock();
| > | > + }
| > | > + return 0;
| > | > +}
| > |
| > | Just a random thought: does this lock really do anything useful here?
| > |
| > Reading the (shared) 'ccids' array is the solution chosen to check whether
| > the module for CCID with number 'id' is already loaded.
| >
| > It would be bad to call request_module() each time a new DCCP socket is
| > opened. Using the 'ccids' array may not be the only way to check whether
| > a given module (whose name depends on the value of 'id') is loaded.
| >
| > But if this solution is chosen, then it requires to protect the read
| > access to 'ccids', which is shared among all DCCP sockets.
|
| Since the lock is dropped after checking ccids[id] then there's
| a window where multiple request_module()s can be called if multiple
| applications create a DCCP socket at a same time. The code below
| should do the same without a lock (ccids is a static array,
| so ccids[N] is always at the same place).
|
| static int ccid_request_module(u8 id)
| {
| if (!in_atomic()) {
| rmb();
| if (ccids[id] == NULL)
| return request_module("net-dccp-ccid-%d", id);
| }
| return 0;
| }
|
Sorry Michael, but this is really just a "random thought". What you are
in effect saying is that reader/writer locks can be replaced with just a
read memory barrier.
Please have a more detailed look at net/dccp/ccid.c. I also checked how
other subsystems handle comparable situations of module loading: the
implementation details differ, but the principle is the same: there are
mutexes, semaphores, and spinlocks in use to protect those shared
structures that are related to the loaded module.
Hence your suggestion does not improve the code. I maintain that it is
correct. And it has proven to work in the test tree for more than one
year, including tests with up to 100 parallel (iperf) connections.
--
To unsubscribe from this list: send the line "unsubscribe netdev" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html| david | Re: Dual-Licensing Linux Kernel with GPL V2 and GPL V3 |
| David Woodhouse | [GIT *] Allow request_firmware() to be satisfied from in-kernel, use it in more dr... |
| Philipp Marek | Re: sys_chroot+sys_fchdir Fix |
| Greg Kroah-Hartman | [PATCH 008/196] Chinese: add translation of volatile-considered-harmful.txt |
git: | |
| Krishna Kumar | [PATCH 9/10 REV5] [IPoIB] Implement batching |
| Gerrit Renker | [PATCH 15/37] dccp: Set per-connection CCIDs via socket options |
| David Miller | Re: [PATCH] pkt_sched: Destroy gen estimators under rtnl_lock(). |
| David Miller | [GIT]: Networking |
